1. Field of the Invention
The present invention relates to a method of protection of an underground structure in ground which has a high water permeability and easily liquefies, such as sandy ground, and particularly to a protection method which is suitable to prevent floating of the underground structure and outflow or wash out of ground from under the structure.
2. Description of the Prior Art
Generally, ground which has a high water permeability and contains excessive pore water easily liquefies when the pressure of the water is increased by an earthquake. Further, an underground structure whose interior is hollow, like an underground tunnel, generally has a small apparent specific gravity. Accordingly, such underground structure constructed in such ground in the past frequently has been floated when liquefaction of the ground was caused by earthquakes.
As a countermeasure to solve such a problem, there are known a method of improving the ground, a method of forming a gravel drain and a method of lowering of the ground-water level. However, to employ these methods, various conditions of the ground and restrictions on long term maintenance and management of the structures would be required. Thus, these methods are not frequently adopted.
It is possible to apply to such type of underground structure additional structures for preventing the ground from being in a liquefied state, e.g. as disclosed in Japanese Provisional Patent Publication No. 127823/1983. However, such additional structures inevitably are accompanied by the following problems. Namely, in such structure, a predetermined number of drain pillars are provided in the base ground to support an underground structure. The drain pillars each comprise a concrete pillar having water permeability. On the outer circumference of the concrete pillar is formed a wall having water permeability with materials such as gravel, fine stone and slag. By this arrangement, it is attempted to employ the drain pillars to prevent an increase of the pressure of excessive pore water in the ground. However, it is difficult to construct such drain pillars under the underground structure, and even if possible, such construction requires a substantially high cost. Moreover, in the above-mentioned structure where the drain pillars are formed under the underground structure, the underground structure is liable to be floated by the pressure of water which elevates in the drain pillars when the ground becomes liquefied.
3. Object of the Invention
The present invention has been accomplished in an attempt to solve the above problems of the prior art. Therefore, it is an object of the invention to provide a method of protecting an underground structure in ground which is liable to be liquefied. Moreover, the method can prevent floating of the underground structure and can be achieved by a constructional operation that can be done easily and at low cost.